setTimeout(function(){
    var a = 'hello'
    setTimeout(function(){
       var b = 'lagou'
       setTimeout(function(){
          var c = 'I ❤ U'
          console.log(a+b+c)
       },10)
    },10)
 },10)

 function fun(msg){
     return new Promise((resolved,rejected)=>{
         setTimeout(()=>{
             resolved(msg)
         },10)
     })
 }
 fun().then(function(){
     return fun("hello")
 }).then(value=>{
     return fun(value+"lagou")
 }).then(value=>{
    return fun(value+"I ❤ u")
 }).then(value=>{
     console.log(value)
 })